Ozone depletion: cause and effect

Ozone formation

1) UV splits diatomic oxygen into 2 monoatomic oxygens

2) Monoatomic and diatomic oxygen join to form triatomic ozone

3) UV splits ozone

These reactions are at a dynamic equilibrium

Effects of UVB light on living organisms

• Any UVB reaching earths surface is absorbed by cells

• The energy of the UVB is converted to chemical energy as it breaks up biological molecules

• Causes skin damage, DNA damage, skin cancer, cataracts, leaf tissue damage, reduced photosynthesis, and damage to marine organisms such as algae, corals and planktonic organisms

Effects of UVB on gases in the atmosphere

• Absorbed/ utilised by diatomic and triatomic ozone on photolysis reactions where the molecule is split

• This prevents most of it reaching earths surface

UV absorption

UVA:

• Wavelength: 320-400 nanometers

• Not absorbed by ozone

• Impacts:

  • All forms of skin aging (wrinkles)

  • Damage collagen and elastin in skin

  • Generates free radicals

UVB:

• Wavelength: 280-370 nanometers

• Almost fully absorbed by ozone

• Impacts:

  • Skin damage, DNA damage, skin cancer, cataracts, leaf tissue damage, reduced photosynthesis, damage to marine organisms

UVC:

• Wavelength: less than 280 nanometers

• Completely absorbed by ozone and oxygen

• Impacts;

  • UVC radiation can cause severe burns of the skin and eye injuries

Ozone depletion

• Since the 1970’s, ozone depletion has been following 3 main patterns

• 4% reduction in ozone per decade

• Ozone holes in polar regions where there is an annual drop in stratospheric ozone concentration

• Concentration at arctic has dropped by 1/3, Antarctic dropped by 2/3

CFCs

• Chlorine, fluorine and carbon

• Developed in 1970’s for use in fridges

• Not flammable

• Non toxic

• Later used as aerosol propellants, solvents and polystyrene

Rowland- molina hypothesis 1974:

• Persistence of CFC’s: very stable so remain in atmosphere long enough to be carried up into stratosphere

• Dissociation and release of chlorine: absorb UV in the stratosphere which breaks bonds with chlorine and frees chlorine free radicals

• Reaction of chlorine and oxygen: choline reacts with monoatomic oxygen, preventing reaction with O2 to form O3. Further reactions prevent formation of more ozone molecules

• Other halogens: bromine and iodine can cause similar reactions

Chlorine and ozone

• Halogens (especially chlorine) in the stratosphere cause ozone depletion

• A single chlorine atom is a radical as it has an unpaired electron, making it highly reactive

Chlorine reactions:

Chlorine + ozone —> chlorine monoxide + oxygen

Chlorine monoxide + oxygen —> Chlorine dioxide

Chlorine breaks down into chlorine + oxygen

• The chlorine is now free to repeat steps again- reducing amount of ozone in the atmosphere

Measuring ozone

• Measured in Dobson units and ,ensured by thickness estimates of the total layer if it existed as a layer at sea level

• 100 DU is equal to 1mm thick

• Normal ozone levels are 300-330 DU

• A hole in ozone refers to levels than 220 DU

• 4 methods of measuring

1) Ground based data collection

• First collected by British Antarctic survey at the Halley station near the southern pole

• Detection of higher levels of UV

2) Satellite surveys

• Satellites in orbit don’t pass through he stratosphere so UV passing through cant be measured

• Readings of UV light reflected by the earth are measured, if these are higher this suggests ozone is reduced

3) Air samples from the stratosphere

• Collected by helium balloons and high flying research aircraft

• Confirmed the chemicals which were causing ozone depletion

4) variability in ozone levels

• variability in ozone at different times, areas and altitudes

• Most severe depletion 12-24km where UV splits ozone

• Worst depletion over Antarctica, levels have dropped below 100 DU

• Depletion in arctic is less severe, globally there has been a drop of 4%

• Severity depends on time of year, in Antarctica the ozone hole is worst between September and December

Ozone depletion over Antarctica

• Stratosphere is much colder over Antarctica

• Winds around Antarctica produce a polar vortex with little mixing with the rest of the atmosphere, isolating Cl

• This allows polar stratospheric clouds (PCS) and ice crystals to form

• These crystals act as a surface for the reactions of chlorine to take place on

• The ice crystals act as a store for the chlorine molecule

• Completely darl in the Antarctic winter, when spring arrives chorine molecules are split

Main aspects of the Montreal protocol

• Manufacture and use of CFCs phased out and banned

• Use of HCFCs will be phased out by 2030

• Some essential uses such as halon fire extinguishes still allowed in aircraft’s

• A fund is available to help the implementation of the protocol

Implementation of the Montreal protocol

Use of alternative processes:

• Pump action sprays

• Stick or roll on deodorants

Use of alternative materials:

• HCFCs replace CFCs, in refrigerators and air con, they are less chemically stable so will break down in the troposphere

• HFCs replace HCFCs as they contain no chlorine

• CFCs as aerosol propellants has been replaced with propane and butane

• CFCs in foam plastics been replaced with HCFCs and HFCs

• CFCs in asthma inhalers gave been replaced with HFAs

• Alternatives to CFC solvents

• Safe disposal waste of CFCs

Evaluation of ulcers of the Montreal protocol

Success:

• International recognition for the serious consequences of ozone depletion

• Agreement from nearly every country that action must be taken

• Development of alternative processes and phasing out of ozone depleting substances

Limitation:

• Use of HCFCs as a replacement adds to enhanced greenhouse effect